Cubilin is a peripheral membrane protein that works in conjunction with its co-receptor megalin to mediate endocytosis of high density lipoproteins (HDL) in the yolk sac visceral endoderm (VE) and in renal proximal tubules. The significance of cubilin mediated HDL uptake to embryonic development as well as adult physiology is not known. During development, it is hypothesized that cubilin activity is essential for embryonic acquisition of maternal HDL-associated cholesterol, phospholipids, triglycerides and/or lipid soluble vitamins required to support embryogenesis. The vital role served by yolk sac cubilin is indicated by the severe developmental abnormalities observed when cubilin antibodies are administered to pregnant rats or to embryos cultured ex utero. In adult physiology, it is hypothesized that renal cubilin/megalin uptake of HDL is involved in a feedback mechanism that influences steady state levels of HDL in the blood. Support for this hypothesis comes from our findings that conditional deficiency of renal megalin activity causes an increase in plasma HDL. To test the above hypotheses, and to characterize fundamental mechanistic features of cubilin function, three specific aims are proposed. The first aim is to characterize the developmental and physiological abnormalities that arise in mice homozygous and heterozygous for targeted deletion of the cubilin gene. Emphasis will be placed on determining of the impact of cubilin deficiency on HDL metabolism. For use in these studies we have developed a novel mouse model of cubilin deficiency. There is also a plan to develop a conditional knockout model to characterize the renal-specific functions of cubilin. The second aim is to determine the significance of each of the three known cubilin co-receptors, megalin, amnionless (AMN) and the cation-independent mannose 6-phosphate/insulin-like growth factor ll-receptor (CIMPR), to the process of embryonic uptake of maternal HDL. These studies will include evaluation of yolk sac endodermal trafficking of HDL to determine whether it is targeted to lysosomes or transported across the VE via transcytosis. The third aim is to define the molecular basis for ligand binding and endocytotic functions of cubilin with the primary objectives being to map binding sites within cubilin for HDL, apolipoprotein A-l and the co-receptors. Together, these studies will provide new insights into the function of cubilin in maternal-embryonic.lipoprotein transport, embryonic development and adult kidney function.